Method and mold to control optical device polymerization

ABSTRACT

A method and mold assembly to control the polymerization of a cast optical device. The mold assembly includes at least one mold portion having a non-critical surface with fresnel or diffractive geometry. Alternately, a fresnel lens or diffractive lens may be placed at a predetermined distance from the mold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Ser. No. 60/193,906which was filed Mar. 31, 2000.

BACKGROUND OF THE INVENTION

The present invention is directed toward controlled curing of devicesrequiring optical cure. More specifically, the present inventionprovides a method for curing optical devices such that the devicesundergo a more uniform polymerization, resulting in a reduction indefects such as dimpling and warpage in the cured device. In particular,the optical devices include ophthalmic lenses including contact lenses,intraocular lenses, spectacle lenses, corneal onlays and corneal inlays.More particularly, this method provides for a method to produce contactlenses having a controlled cure profile.

It is often desirable to direct-mold optical devices such as contactlenses and intraocular lenses, rather than form the lenses by machiningoperations. In general, molded lenses are formed by depositing a curableliquid such as a polymerizable monomer into a mold cavity, curing theliquid into a solid state, opening the mold cavity and removing thelens. In particular, the mold cavity may be formed by a mold assemblycomprised of a posterior mold portion and an anterior mold portion, eachhaving a lens-forming surface. When the posterior mold portion andanterior mold portion are mated, the lens-forming surface of theposterior mold portion and the lens-forming surface of the anterior moldportion form the lens-forming cavity. The non-lens-forming surfaces ofboth mold portions, herein referred to as non-critical surfaces, aregenerally molded to have a similar radius (or radii) of curvature asthat of the lens-forming surfaces. While the lens-forming surfaces areof optical quality, each having a central optical zone and typically, atleast one peripheral carrier zone, the only requirement of thenon-critical surface generally is a smooth surface.

Polymerization is typically carried out by thermal means, irradiation orcombinations thereof. Traditionally, conventional thermo-castingtechniques require fairly long curing times and are used when theresultant object is thick. Rods from which rigid gas permeable lensesare lathed from or thicker lenses are often thermally cured. Curing oflenses by irradiation, in particular, ultraviolet (UV) irradiation,frequently offers shorter curing times. The monomer is poured into atransparent mold having a desired optical surface, and thereafter the UVenergy irradiates the monomer through the transparent mold to cure thephotosetting monomer.

A common material used as a mold material is polypropylene, which isdisclosed in U.S. Pat. No. 5,271,875 (Appleton et al., assigned toBausch & Lomb Incorporated, the entire contents herein incorporated byreference). The process disclosed in Appleton et al., may be used toproduce lenses with predictable and repeatable characteristics.

The use of polypropylene may be desired with certain lens-formingmaterials. Other lens-forming materials, however, may cast just as wellor better in other mold materials. As disclosed in U.S. Ser. No.09/312105 (Ruscio et al. and assigned to Bausch & Lomb Incorporated, theentire contents herein incorporated by reference), polyvinyl chlorideabsent any UV stabilizer provides a suitable material for the posteriormold.

While the irradiation of the optical device from the light source may beconducted in a uniform and parallel manner, the material chosen for themold portions may affect the pathways of the light rays. For instance,some materials, such as thermoplastic crystalline polymers, may diffusethe radiation, causing a scattering of the light rays. Polypropylene issuch a material. Other materials such as polyvinyl chloride andpolystyrene are thermoplastic amorphous polymers, which permit anunhindered pathway for the light rays during curing.

The radiation may also be reflected off the surface of the glass orplastic mold materials. This may result in non-uniform distribution oflight intensity over the lens-forming material.

This invention recognized that the non-critical surface of the posteriormold may act as an optical device, reflecting and/or refracting theradiation in a non-uniform pathway through the mold portion. Inparticular, the non-critical surface of the mold may refract theradiation from the optical source. This may lead to non-uniform curingrates of ultraviolet polymerizable materials including resins andmonomers. As a result, since the curing is completed faster and morecompletely in a portion receiving a high radiation intensity (in thisinstance, the periphery portion of the lens) and slower in a portionreceiving a low radiation intensity (the central portion), stress isgenerated in the cured resin or monomer layer. This stress maydeteriorate the precision of the optical device face. Additionally, thefaster curable portion receiving higher radiation intensity is curedwith absorption of the surrounding uncured resin or monomer in order tocompensate for the contraction of resin or monomer resulting from thecuring process. As a result, the slower curable portion (which receiveslower radiation intensity) may show defects such as shrinkage. Inparticular, in the case of contact lenses and spectacle lenses, this canproduce lenses with unacceptable optical aberrations caused by unevencuring and stress. “Dimpling” or warpage of the contact lens is a commonproblem caused by uneven curing. In dimpling, the apex of the lens isflattened or slightly concave in shape. Warpage is generally seen as theinability of the edge of a lens to have continuous contact with themolding surface upon which it contacts. Other drawbacks seen withplastic spectacle lenses include “striations”, which are caused byuneven curing and stress. Thermal gradients form in the gel-state, whichproduce convection lines (“striations”) that become frozen in place andcannot be dispersed.

U.S. Pat. No. 4,166,088 (Neefe) discloses controlling the polymerizationof cast optical (plastic or contact) lenses. The mold section on thebottom is a lens which focuses UV light to the center of the cavity. Thebottom mold must have a thickness which corresponds to the focal lengthof the refractive surface so that the UV light rays converge at thecenter of the monomer being cured. Neefe also requires an aluminumreflector on the outer surface of the top mold to reflect light backthrough the monomer.

U.S. Pat. No. 4,534,915 (Neefe) discloses the use of a convex positiverefractive power cylinder lens to provide a band of actinic light to arotating lens monomer. The center of the spin cast lens receives themost radiation, the area adjacent to the center receives less while theperiphery receives still less radiation. This allows for the outerportion of the spin cast lens to migrate inward as the lens shrinksduring the curing process. A fresnel lens or a Maddox rod may also beused to provide the narrow high energy line of actinic light.

U.S. Pat. No. 4,879,318 (Lipscomb et al.) discloses the use of moldmembers formed from any suitable material that will permit UV light raysto pass through. To aid in the even distribution of the UV light, thesurfaces of the molds are frosted. In one embodiment, a Pyrex glassplate is used to filter out UV light below a certain wavelength.Lipscomb et al. found that if incident UV light is not uniformthroughout the lens, visible distortion pattern may appear in thefinished lens. Lipscomb et al. solved this problem by includingadditives in the lens forming composition to reduce the distortions. Theophthalmic lenses are formed from plastic.

U.S. Pat. No. 4,919,850 (Blum et al.) discloses a method for makingplastic lenses in which the liquid lens material is dispensed into themold cavity and put into a heated bath for a partial thermal curing.After a period of time, the mold (while still in the liquid bath) issubjected to UV light for an additional period of time. The liquid bathdisperses the UV light sufficiently to avoid stresses and other adverseeffects on the lens ultimately formed that may be caused by unevenexposure to the UV light. The mold may also be rotated while in the bathor the bath may include an aerator to enhance the dispersion of the UVrays. By rotation of the mold and aeration of the bath, the surface ofthe mold is also kept free of any debris which may otherwise channel theUV light. Additionally, a reflective surface provided on the one of themolds forms may reflect UV light back through the lens material beingcured.

U.S. Pat. No. 4,988,274 (Kenmochi) discloses irradiating the centralportion of the mold cavity containing the lens-forming material toinitiate a photocuring reaction. The area of the light, in the shape ofa ring, is enlarged until the lighted area reaches the periphery of thelens-forming material. A variable power lens, including a fresnel lens,may be used to align the light. The lens-forming material in the centerof the mold cavity is cured first which causes the lens-forming materialaround it to shrink. The shrunk volume of lens-forming material issupplemented with additional uncured lens-forming material. The variablepower lens allows for adjustment of the ring-shaped light.

U.S. Pat. No. 5,135,685 (Masuhara et al.) discloses the use of aconveyor or other moving device to continuously move objects to beirradiated by a multiplicity of aligned sources of visible light. Themovement of the irradiated objects may be linear or curved movement onthe same plane or upward or downward movement.

U.S. Pat. No. 5,269,867 (Arai) discloses a method for producing glasslenses with a coating on one side. The coating is a resin layer that iscured with UV light. The resin is dropped onto a metal mold (with areflective surface) and the glass lens placed on the resin. The resin isinterposed between the lens and the metal mold. UV light is providedthrough the glass lens, curing the resin. A filter may be used to evenlydistribute the UV light. Without the filter, the reflection of the metalmold and the glass lens result in non-uniform distribution of UV lightand non-uniform curing speed. The center of the resin cures faster thanthe outer perimeter, causing defects such as shrinkage in the resin.

U.S. Pat. No. 5,529,728 (Buazza et al.,) discloses a method of curing aplastic eyeglass lens. The method comprises placing a liquidpolymerizable composition within a mold cavity defined by mold membersand a gasket. A first set of UV rays are directed to one of the moldmembers. The gasket is removed and a second set of UV rays is directedto the lens. Buazza et al., further discloses the use of a filter whichincludes a plate of Pyrex glass to diffuse the UV light so that it hasno sharp intensity discontinuities. To produce a positive lens, the UVlight intensity is reduced at the edge portion so that the thickercenter portion of the lens polymerizes faster than the thinner edge ofthe lens. Mold members of Buazza et al., are preferably precision groundglass optical surfaces having UV light transmission characteristicsincluding casting surfaces with no surface aberrations, waves, scratchesor other defects.

None of the above art completely solves the problems which occur whenusing a mold assembly in which one mold portion or both molds is madefrom an amorphous material and acts as an optical device. The resultantlens made from this particular mold assembly may have defects such asdimpling and warpage.

SUMMARY OF THE INVENTION

The present invention is a method for photocuring cast articles such asophthalmic lenses in which defects in the cured article are reduced. Byaltering the pathway by which radiation rays reach the article to becured, defects can be reduced. By controlling the relative intensity ofradiation upon a particular portion of lens-forming material, the rateof polymerization taking place at various portions of the lens can becontrolled. While this method works well with crystalline materials, itis particularly suited for use with mold materials which are amorphous.

In this invention, the radiation path from a source is altered so as toobtain a desired cure profile across the mold cavity. Nearly any curedprofile may be attained including non-axisymmetric profiles. Thisresults in a desired cure gradient across the cast article. Theradiation path may be altered in various ways, including the use of anoptical element or an optical surface cut into the non-critical surfaceof the mold receiving radiation.

In the preferred embodiment, fresnel or diffractive surfaces may be usedto alter the radiation pathway. In particular, the non-critical surfaceof a mold may have fresnel or diffractive geometry formed into it. Theresultant geometry of the non-critical surface may allow the mold to actas a negative, positive or neutral lens.

Alternately, a fresnel or diffractive lens may be placed between thelight source and the mold. The lens may be placed at a predetermineddistance away from the mold. The lens will alter the path of radiation,preferably ultraviolet (UV) radiation, passing through the mold andincrease the energy available to the cured article. As a result, thedistribution of radiation or energy across the mold will have a desiredprofile, which may remove any residual stress induced during curing. Theresult is a cured article such as a contact lens having an acceptableapex in the central portion of the lens. The optical element allowscontrol of the illumination intensity profile reaching various sectionsof the contact lens. Unwanted stress induced by uncontrolled intensityprofiles can be corrected. In addition, stress can be introduced inspecific amounts and locations throughout the cast article as desired.

The ophthalmic lenses formed from these methods are relatively free fromdefects such as dimpling and warpage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional elevational view of a posterior mold sectionassembled with an anterior mold section;

FIG. 2 is a perspective exploded view of a mold assembly including acontact lens;

FIG. 3 is a cross-sectional elevational view of a posterior mold sectionshowing light diffusion through the mold section; and

FIG. 4 is a cross-sectional elevational view showing light diffusionthrough a posterior mold section having fresnel geometry.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is useful for the method of photocuring castarticles such as ophthalmic lenses. While this invention may be used toproduce any device produced by photocuring, preferred embodimentsinclude the method of making intraocular and contact lenses.

As seen in FIGS. 1 and 2, mold assembly 5 defines mold cavity 40 forcasting lens 30, including anterior mold portion 10 for defining theanterior lens surface 32 and posterior mold portion 20 for defining theposterior lens surface 34. Anterior mold 10 has lens-forming surface(critical surface) 12 and opposing non-critical surface 14. Posteriormold 20 has lens forming surface 22 and opposing non-critical surface24. When posterior mold section 20 is assembled with an anterior moldsection 10, lens-forming cavity 40 is formed between posterior moldsection lens forming surface 22 and anterior mold section lens-formingsurface 12. As discussed in Appleton et. al., lens 30 formed from thismold assembly include a central optical zone 42 and a peripheral carrierzone 44. The peripheral zone 44 has a substantially greater volume thanthe optical zone 42 and may include a tapered edge.

As illustrated in FIG. 1, rays 25 from optical source 1 irradiatenon-critical surface 24 of posterior mold portion 20. The indices ofrefraction encountered by rays 25 change as the rays pass through airand then through a solid material.

The preferred materials for posterior mold portion 20 is an amorphousmaterial such as polyvinyl chloride (PVC), polystyrene, an amorphouscopolymer of ethylene and a cyclic olefin (such as a resin availableunder the tradename Topas from Hoechst Celanese Corporation) but mayinclude crystalline thermoplastic materials as well (includingpolypropylene). Other suitable materials include inorganic standardglass, glasses available under the tradename Pyrex, borosilicates,polymethyl methacrylate, polycarbonate, acrylonitrile copolymer (such asresin available under the tradename of Barex), TPX (4-methyl 1-pentene)and polyacrylonitrile. Accordingly, it is preferred that anterior mold10 is amorphous but other materials such as metals and crystallinethermoplastic materials may be used as well.

The optical or radiation source may be actinic, electron beam, laser orradioactive source, but is preferably ultraviolet lamps which irradiatesthe monomer. Visible light or infra-red light may also be used.Radiation may also be from a high intensity UV source. Additionally,combinations of light irradiation and thermal means may be used. Unlessspecified, the term “light” will refer to any actinic wavelength orrange of wavelengths.

Posterior mold 20 can further be described by its optical parameters. Inparticular, based on the parameters of a posterior mold used to producecommercially available lenses but using an amorphous material such asPVC, one can calculate the powers of each surface of the mold:non-critical surface radius of −6.0 mm (R₁), critical surface radius of−8.0 mm (R₂), index of refraction of PVC mold material of 1.5 (n₂),index of refraction of air of 1.0 (n₁), center of thickness of the moldof 2.0 mm (t) and index of refraction of lens-forming monomer of 1.4(n₃). While HEMA (2-hydroxyethylmethacrylate) is a preferred monomer,any lens-forming material may be used. Especially preferred arematerials that are capable of free radical polymerization. Preferredmaterials include silicone and methacrylate hydrogels. Preferredexamples of applicable materials are disclosed in U.S. Pat. Nos.5,610,252 and 5,070,215 (Bambury et al., assigned to Bausch & LombIncorporated, the entire contents herewith incorporated by reference).

The posterior mold is a negative lens with essentially all of itsnegative power coming from the non-critical surface. The negative powerof the mold causes incident UV rays to diverge as they pass through themold which leads to a reduction in intensity at the center of thelens-forming cavity. The power of the posterior mold can be described bythe following equations:

Power of non-critical surface 24 of posterior mold 20:

φ₁=(n ₂ −n ₁)/R ₁=−83.333D

Power of critical (lens-forming) surface 22 of posterior mold 20:

φ₂=(n ₃ −n ₂)/R ₂=+12.5000D

The total power (Φ) of mold posterior:

Φ=φ₁φ₂−(t/n ₂)φ₁φ₂=−69.444 D

Non-critical surface 24 of posterior mold 20 is typically spherical witha radius of curvature that is concentric with equivalent radii oflens-forming surface 22. This keeps the thickness relatively constantacross the posterior mold. This concentric requirement forces posteriormold 20, especially when posterior mold 20 is an amorphous material, tobe a substantially negative lens. As illustrated in FIG. 3, rays 25passing through non-critical surface 24 of posterior mold 20 arerefracted outward, away from the center optical portion (not shown) andtoward the peripheral carrier zone of the lens (not shown) being cured.

A positive lens can be used to reduce the negative power of the mold.One embodiment of this invention may be the use of a lens with fresnelor diffractive components to direct radiation to a certain area. Afresnel lens or diffractive lens can be placed above the posterior mold.This would involve placing a lens at a predetermined distance from theposterior mold, thereby causing the light rays to converge toward theoptical zone of the lens.

Alternately, a fresnel lens or diffractive lens may be incorporated intonon-critical mold surface 224 of posterior mold 220 as shown in FIG. 4.In this manner, non-critical surface 224 can be altered such that thenon-critical surface can achieve nearly any optical power profile. Thus,stress can be induced to any portion of the lens as desired.Non-critical surface 224 may be formed by laser or direct machining,compression/injection molding, etching or any other applicable method.

For example, non-critical surface 224 can be made to have zero opticalpower. This can be accomplished with “staircase” geometry as shown inFIG. 4.

Alternately, a fresnel or diffractive surface may be cut into thenon-critical surface of the anterior mold. For instance, convexnon-critical surface 14 of anterior mold 10 shown in FIG. 1 may have afresnel or diffractive surface. This would be appropriate if the opticalsource irrradiated the anterior mold.

The power of a typical mold is calculated previously. It is shown thatthe powers of the non-critical and critical surfaces are −83.3 D and+12.5 D, respectively. By forming flat steps into the non-criticalsurface (i.e., the optical radius is now infinity), the power of thenon-critical surface can be reduced to zero. In this case, the totalpower of the mold is +12.5 D. For example:

Power of non-critical surface:

φ₁=(n ₂ −n ₁)/R ₁=(1.5−1.0)/infinity=0

Power of critical surface:

φ₂=(n ₃ −n ₂)/R ₂=(1.4−1.5)/(−0.008 m)=+12.5000 D

The total power (Φ) of the mold is:

Φ=φ₁+φ₂−(t/n ₂)φ₁φ₂=0+12.5D−(0.002 m/1.5) (0)(12.5D)=+12.5 D

An advantage of incorporating a fresnel surface into the non-criticalsurface of the posterior mold is that the resulting mold would haveapproximately the same thickness profile throughout. In makinghigh-precision molds by an injection molding process, it is generallypreferred that the molds have a uniform thickness.

Another benefit of modifying the non-critical surface of the posteriormold, rather than using an external lens, is the posterior mold willaccept a wider cone of radiation, thereby making it more efficient.

In an alternate embodiment, one may choose to introduce predeterminedstress profiles into a lens, rather than remove them. In a specificinstance, it may be desirable to form a lens having a specific shapewhich would alter the fitting of the lens to the eye, such as increasingthe lens movement when worn on the eye. A specific parameter which couldbe stress-induced is edge lift which causes the edge of the lens to beslightly raised off the eye. Inducing stress to a lens can be performedby altering the parameters of the optical devices altering the radiationpathway.

The following example serves to illustrates the use of fresnel geometryincorporated into the non-critical surface of the posterior mold.

EXAMPLE 1

A series of HEMA lenses were cast according to conventional methods withthe posterior mold portions prepared from an amorphous resin. Two setsof posterior mold portions had fresnel geometry cut into thenon-critical surface with resulting hydrated lens powers of −6 D and −2D. Two sets of posterior mold portions (also yielding hydrated lenspowers of −2 D and −6 D) had controlled geometry (no junctions) andserved as controls. After casting, the lenses were hydrated andparameters were measured.

TABLE 1 Posterior Mold SAG (mm) −2D Control 3.581 −2D Fresnel 3.581 −6DControl 3.382 −6D Fresnel 3.420

The −6 D lenses made with the control molds failed to meet sagrequirements because of dimpling/warpage. The −6 D lenses made with theFresnel molds met the sag specification and did not have dimples. Otherlens parameter requirements, although not listed, were also met by the−6 D lenses made with the Fresnel molds.

I claim:
 1. A mold assembly for molding optical devices comprising afirst mold portion and a second mold portion, said first mold portionhaving a lens forming surface and an opposing non-critical surface witha distance therebetween defining a thickness which is substantiallyconstant over the area of said surfaces, said non-critical surfacehaving fresnel geometry.
 2. The mold assembly of claim 1 wherein saidfirst mold portion is comprised of an amorphous material.
 3. The moldassembly of claim 1, wherein said first mold portion is comprised of acrystalline material.
 4. The mold assembly of claim 1, wherein saidnon-critical surface comprises zero power.
 5. The mold assembly of claim1, wherein the first mold portion is comprised of polyvinyl chloride. 6.The mold assembly of claim 1, wherein said lens forming surface isshaped to form a contact lens.
 7. The mold assembly of claim 1, whereinsaid lens forming surface is shaped to form an intraocular lens.
 8. Themold assembly of claim 1, wherein said first mold portion is a posteriormold wherein said lens forming surface is convex and said non-criticalsurface is concave.
 9. The mold assembly of claim 1, wherein said firstmold portion is an anterior mold portion wherein said lens formingsurface is concave and said non-critical surface is convex.
 10. The moldassembly of claim 1, wherein said fresnel geometry of said non-criticalsurface is formed by injection molding.
 11. The mold assembly of claim1, wherein said fresnel geometry of said non-critical surface is formedby direct machining.